The basic design concept about axial flux motors had been around for more than a century, in which the axial flux motor with a stator core that is made of a magnetic conductive material is the motor most often being adapted for applications requiring direct drive and large torque since it has comparatively larger air-gap flux density for achieving higher torque density more easily. However, three are still many technical difficulties to be resolved just to be able to use common silicon steel for manufacturing stator cores for axial flux motors in mass production. One major technical difficulty is that: during the making of a stator core out of a silicon steel strip, since the silicon steel strip is being stamped with teeth and slots and the same time is being spirally wound, the spiral winding and the teeth stamping must be coordinated and controlled accurately for allowing teeth and slots to be formed with continuously varying pitches, and thereby, enabling those teeth or slots of different layers to be aligned with one another precisely so as to ensure a good slot straightness. Thus, a stator core with good slot straightness not only can ensure a specific slot fill factor to be maintained, but also it is helpful for allowing a pre-wound coil to mount smoothly onto its corresponding slot.
In addition, for reducing clogging in permanent-magnet axial-flux motors with wound stator core and/or for decreasing harmonic ratio in power generators, generally the air-gap flux density distribution is required to be adjusted for enabling the waveform of the corresponding counter electromotive force to resemble a sine wave or decreasing the harmonics, and that adjustment is usually achieved by modifying the top profiles of the teeth in the stator as each tooth is treated and acted as a magnet. Generally, in all the current methods for manufacturing wound stator core for axial-flux motors, there is never a step designed for modifying the top profiles of the teeth in the wound stator core, but instead, the top profiles of the teeth are modified and adjusted by an additional grinding process that is performed after the whole spirally winding of the stator core is completed. However, the use of the additional grinding process for cutting and removing excess materials from each tooth so as to shape the top profiles of the teeth into required shapes can be disadvantageous in that: in addition to the increase in material loss, the additional grinding process is going to cost more in energy consumption and work hour. Thus, a feasible mass production method for manufacturing high-torque direct-drive axial-flux motors is a method that can reduce the amount of waste material to be generated, reduce the amount of process required to be performed for making a stator core, and increase the rate of production, while the same time allowing teeth of various top characteristics to be formed as desire at will.
In order to fully utilize the available space inside the slim-type motor while also improve its operation efficiency, the design as well as the method of using electrical steel to manufacture stators is one of the key factors. Since most motors are primarily made of electrical steel and copper wires which together can take up more than 40% of the their manufacture cost, how to balance between cost and operation efficiency is an importance issue to be considered in stator design and development. Notably, the major energy losses in stator core are iron loss and stray loss, which are closely related to the material characteristic of the electrical steel used for building the stator, the stator size and also the process accuracy for making the stator. Generally, early stators and rotors are made directly from a block of steel, and then with the improvement of technology, they are formed by lamination of steel sheets. Nevertheless, no matter they are formed by the processing of a block of steel or by lamination of steel sheets, the common drawbacks in the two processes are high energy consumption, high material loss and limited by one-time operation. Nowadays, most stators are formed by a means of continuous lamination, using which a silicon steel strip is being stamped continuously and the same time is being spirally wound into a stator core. Such stator cores of continuous lamination had been used in many radial-flux motors by major motor manufacturers in Europe, America and Japan, and are also becoming more and more common in axial-flux motors. However, since the stray loss as well as motor noise are directly related to the compactness of the stator lamination, the winding tightness and alignment accuracy in the spirally winding process are keys to build a good stator core.
In addition, the other means of adopting high-grade silicon steel sheet with reduced thickness in a process of high machining accuracy and optimal design is also effective for achieving a good stator core with low stray loss and low noise. However, not to mention that the high-grade silicon steel sheet with low stray loss is generally very expensive, the current process for stamping and spirally winding such high-grade silicon steel sheet is a process of high energy consumption and high material consumption. Although the use of silicon steel sheet with reduced thickness can effectively reduce the amount of material wasted in the machining process, the material strength of such thin silicon steel sheet is generally not sufficient enough that certain deformations such as curling deformation to the teeth of stator lamination can be caused during the stamping of the silicon steel sheet. Responsively, for solving such deformation caused by insufficient strength, one method is to attach an addition metal strip to the flat side of the teeth tops in the stator lamination to be used as connection anchor, but it is going to cause more material cost and difficulty in machining. On the other hand, there is another method that can effectively solve the aforesaid deformation problems by stamping teeth and slots in a stator lamination in an alternating manner, but it is disadvantageous in that: the shape of the teeth and slots to be formed in the stator lamination as well as the available space in the stator lamination will further be limited by the alternating design.
Moreover, as the defect rate of stators formed from a spirally winding process is generally high that not only the defective work loss is high, but also the overall production time is prolonged. In addition, although the stators formed from a spirally winding process can be manufactured using a punch stamping press with smaller press tonnage and thus the energy loss in the stamping can be reduced, but the number of punch stamping required for forming just one stator is much more than those stators not formed by a spirally winding process and thus the percentage of wear-and tear to the punch head is higher.